1. Field of the Invention
The present invention relates to an optical transmission module, a wavelength monitor, and a wavelength drift detection method, and more particularly, to a technology for detecting a wavelength drift of laser light.
2. Description of the Related Art
In a wavelength multiplexing optical communication system, wavelength grids are generally set at intervals of as narrow as 50 to 100 GHz, and an optical line is assigned to each wavelength grid. In such a system, there are cases where a wavelength drift from a target wavelength grid of an optical signal, which occurs on a line, affects the communication conditions of not only that line but also other adjacent lines. For this reason, means for detecting whether or not the light wavelength of each line exceeds a predetermined range is necessary.
In this regard, there is known a technology of stabilizing the light wavelength by monitoring the laser temperature by a temperature detector such as a thermistor and keeping the laser temperature constant with the use of a Peltier device or the like. JP 2004-132704 A and JP 2003-324241 A each disclose a wavelength monitor that monitors the light wavelength with high accuracy by using an etalon filter. The etalon filter is an optical filter having such a characteristic that the transmittance periodically increases and decreases with respect to the wavelength (frequency) as illustrated in FIG. 4. As long as the light wavelength varies in the neighborhood of the target wavelength illustrated in FIG. 4 (circle indicated as an own line n), the light wavelength can be correctly estimated by monitoring the transmittance of light output from the etalon filter.
However, in the above-mentioned wavelength monitor disclosed in JP 2004-132704 A or JP 2003-324241 A, there is a problem in that the light wavelength is erroneously estimated when the light wavelength drifts significantly from the target wavelength due to a deterioration or a failure or the like of the optical transmitter. For example, as illustrated in FIG. 4, when the light wavelength of the own line n is drifted to the neighborhood of an adjoining wavelength grid (target wavelength of adjoining line n±1), the transmittance of the light passing through the etalon filter has a value approximately equal to a value of the transmittance when the light wavelength is in the neighborhood of the target wavelength, and hence the wavelength monitor erroneously recognizes that the light wavelength is in the neighborhood of the target wavelength. Consequently, with the above-mentioned wavelength monitor, there occurs a case in which when the light wavelength drifts significantly, the wavelength drift is erroneously detected and communication failure cannot be correctly detected.